For certain applications in electrical machinery, especially those that depend on the current intensity to be transmitted in connection with the installation conditions, there are used drum commutators, also known as cylindrical commutators, in which the brush contact face is disposed on a regular cylinder concentric with the commutator axis. In addition to drum commutators with metal brush contact face there are known several modifications of drum commutators of the type cited in the introduction, in which the brush contact face is disposed on the carbon segments. In a first known design of this type, the carbon segments are molded around the conductor segments. Such a drum commutator as well as a method suitable for producing the same are described, for example, in EP 0529911 B1. WO 99/57797 A1 also describes an analogous drum commutator and a method suitable for producing the same, characterized in particular by the fact that the carbon segments are molded around the conductor segments. The same is true for DE 4241407 A1 and U.S. Pat. No. 5,789,842 A.
According to a fundamentally different design, a carbon shell comprising the subsequent carbon segments is made beforehand independently of the conductor segments, and only later is joined to the latter in electrically conductive relationship. A drum commutator of this design and a method suitable for producing the same are described in DE 3150505 A1. In this case a carbon shell is joined end-to-end in electrically conductive relationship to an annular conductor blank by soldering. A barrel-shaped support body of insulating compression-molding material is then injected into the interior of the corresponding unit. Finally, the carbon shell and the conductor blank are divided into individual segments by axial parting cuts.
It is not known that drum commutators according to DE 3150505 have ever been used successfully. Obviously the drum commutator known from that document is not practical, even though the design is compelling at first sight.
Drum commutators with carbon contact face, in which the carbon segments are molded and then sintered onto the conductor segments as explained hereinabove in connection with EP 0529911 B1 and the publications that are comparable in this regard, have not proved any more effective in practice. In such drum commutators, poor contact between the carbon segments and the associated conductor segments has been consistently observed. In this connection, it must be considered that drum commutators of the type in question here are subjected to extreme operating conditions. For this reason, it is required that they withstand several hundred cycles at operating temperatures of −40° C. to +110° C. without failing under all conceivable ambient conditions (especially the most diverse fuels). In the corresponding strict tests, known drum commutators of the design cited in the introduction consistently exhibit unacceptably high resistances, suggesting poor contact between the carbon segments and the conductor segments, or else they fail completely. One reason for this may be that the wires of the rotor winding, which is attached to the commutator, are routinely welded onto the conductor segments. Because of the very high temperatures occurring during this process, the metal conductor segment in question briefly expands by a not inconsiderable percentage before shrinking once again. Not only does this lead to impairment of the mechanical joint between the carbon segments and the associated conductor segments, but also the electrically conductive joint between those parts suffers commensurately, with the result that the resistance increases. This has a particularly detrimental effect, because the carbon molding compound used to produce carbon segments by molding around the conductor segments has in any case a relatively high binder content (up to 30%), thus leading to reduced conductivity.